With the performance improvement of physical machines (hereinafter also referred to as VM host), research into virtualization technology for consolidating a plurality of virtual machines to one physical machine is in progress in recent years. In the virtualization technology, for example, a virtualization software (hereinafter also referred to as hypervisor) allocates resources of a physical machine to a plurality of virtual machines, and services can be provided by application programs (hereinafter also referred to as application) installed in the virtual machines.
With such progress of virtualization technology, sometimes a virtual machine may execute a mission-critical task that was conventionally performed by a physical machine. Accordingly, the virtual environment of a virtual machine is now desired to have a level of reliability equal to that of the physical environment of a physical machine (see, for example, Japanese Patent No. 5032191, Japanese Translation of PCT Application No. 2004-530196, and Japanese Patent Application Laid-open No. 10-187638).